For the final credits of Jersey Boys, director Clint Eastwood sends the whole cast into a backlot street to dance to the Four Seasons' most recent chart-topper, 1976's "December, 1963 (Oh, What a Night)." Hmmm, the confused viewer might wonder, perhaps this is supposed to be a musical....

The movie is, of course, derived from the long-running Broadway hit, and includes numerous Four Seasons songs. But this adaptation, as reworked by original writers Marshall Brickman and Rick Elice, adds dialogue and subtracts music. Large sections are devoted to the drama of making it, or of keeping it after initial success.

Eastwood may not be the project's ideal director. He's known for working quickly, which could explain why parts of Jersey Boys feel undercooked. (The rear projection in car scenes is particularly cheesy.) But the movie is engagingly lively, if not always graceful, and often surprisingly comic.

The laughs can be incongruous, since the backstory is grim: Future Four Seasons Tommy DeVito (Vincent Piazza), Nick Massi (Michael Lomenda) and Frankie Castelluccio (John Lloyd Young) are inept small-time criminals in a crummy Newark suburb. They're under the protection of a local mob boss (Christopher Walken), a devotee of sentimental ballads, although that doesn't keep Tommy and Nick from doing time.

Such hits as "Walk Like a Man" arrive after Castelluccio renames himself Valli, songwriting prodigy Bob Gaudio (Erich Bergen) joins the lineup, and veteran producer Bob Crewe (Mike Doyle, teetering on the edge of gay caricature) plumps the arrangements. Crewe and Gaudio are also Jersey boys, although the latter is suspect because he's from the right side of the tracks. (Another local, future tough-guy actor Joe Pesci, plays an amusing role in all this.)

Top 40 triumph is scarred by a broken marriage, a fatal drug overdose and massive embezzlement, among other woes. Some of these developments are foreshadowed; others just pop up, as if inserted carelessly from a much longer cut.

The stage musical was divided into four chapters, each narrated by a different Season. The movie keeps that device, but scatters each member's commentary throughout the story. You never know when Frankie, Bob, Tommy or Nick will turn to the camera and start talking to the viewer.

Such direct address is a little jarring, since the movie often aspires to naturalism. Eastwood includes enough F-words to guarantee an R-rating for a movie about G-rated pop, and deploys careful period details and sepia-tinted images to evoke the days before day-glo.

And when was that? Onscreen dates identify early scenes as happening in the 1950s, but most of the story occurs out of time. Aside from an appearance by the Angels, who sing their pre-Beatles "My Boyfriend's Back," there's little sense of context, either social or musical.

By the time the Seasons released "Rag Doll" in 1964, they occupied the same universe as Motown, Bob Dylan and the British Invasion. Yet the movie never hints that the group's doo-wop style had become dated. Still, the fact that the Seasons were older than their Top 40 competitors suits the cast, some of whom have a couple of decades on the teenagers they initially play.

Although Jersey Boys rarely presents an entire Four Seasons number without interruption, the music does propel the movie. The group was known for martial drumbeats as well as Valli's stratospheric falsetto, and the songs still have juice, even when presented in snippets or as background music. But they may not pack enough Jersey swagger to attract moviegoers whose earliest pop memories are of Katy Perry or Justin Bieber.